1. Field of the Invention
The present invention relates to a scanning electron beam exposure system used in the manufacture of semiconductor devices.
2. Description of the Prior Art
In the manufacture of semiconductor devices, conventional optical lithographic technologies have been used for producing circuit patterns on chips. The minimum line width is, in this case, about 2 to 3 .mu.m. However, in order to make large-scale integration circuits more complex, the minimum line width must be in the sub-micrometer region. For this purpose, scanning electron beam exposure systems have been developed.
In the optical lithographic technologies, a 10.times. reticle is manufactured. The reticle is then projected onto a master mask demagnified 10 times. In this case, a large number of the same chip patterns of the reticle are repeatedly projected onto the master mask so as to complete the master mask. However, in the scanning electron beam exposure technologies, the chip patterns are directly depicted on a master mask or a wafer without the manufacture of a 10.times. reticle. Therefore, the turnaround time is reduced, the manufacturing costs are reduced, and the accuracy of the patterns is improved, as compared with conventional optical technologies.
In the electron beam exposure system, since the deflection region of an electron beam is very small as compared with the size of a mask or a wafer, a workstage for carrying the mask or wafer must be moved so as to expose the entire mask or wafer. In the scanning electron beam exposure technologies, there are two methods for moving a workstage; a continuously moving method and a step-and-repeat method.
In the continuously moving method, the electron beam is raster-scanned in one direction, such as the X-direction, and the workstage moves in the other direction, such as the Y-direction, at a definite speed. In this method, the exposure time has no relation to the complexity of the patterns, and, accordingly, there is a large effect in the case of complex patterns. Note that, in the optical depiction of a reticle, the exposure time is dependent upon the complexity of the patterns.
In the step-and-repeat method, the pattern is exposed by deflecting the electron beam over a square field, such as one chip, with the workstage stationary. After exposure, the workstage is moved to a new location, and pattern exposure is repeated. This process continues until the entire mask or wafer is covered. The two scanning or writing methods for one chip are a raster-scanning method and a vector-scanning method. In the raster-scanning method, the electron beam is scanned over the entire chip area and is turned on and off according to the desired pattern. In the vector-scanning method, the electron beam addresses only the pattern regions requiring exposure. Usually, in this case, the pattern is decomposed into a series of simple shapes, such as rectangles and parallelograms. In the vector-scanning method, the exposure time becomes longer when the pattern is more complex. However, there is the possibility of a higher exposure speed than the exposure speed of the raster-scanning method.
The exposure time for one 3-inch (77 mm) mask or wafer according to the scanning electron beam system is about 5 to 40 minutes.
In order to more effectively progress the mass production of semiconductor devices, it has been proposed to add an automatic exchanging and loading function. As a result, when the exposure operation for one mask or wafer is completed, the operation for loading the next mask or wafer is automatically performed, and, after that, the operation for exchanging the exposed mask or wafer for an unexposed mask or wafer is also automatically performed. Therefore, it is possible to exclude from the exposure operation the harmful effects of mechanical vibration or impact or electrical or magnetic noise due to the opening or closing of a vacuum valve, the motion of a holder, or the like.